The present invention, in some embodiments thereof, relates to light emission and, more particularly, but not exclusively, to the emission of light using a bilayer quantum well structure as an active layer.
In a semiconductor light emitting device such as a semiconductor laser and a light emitting diode, electrons and holes injected into an active layer of the device combine with each other and emit light. It is generally desired to confine the electrons and holes in the active layer in order to improve characteristics of the light emitting device. A conventional semiconductor laser has a double heterostructure, in which an active layer is sandwiched between a p-type cladding layer and an n-type cladding layer. Compound semiconductors for forming the double hetero structure are selected so that the forbidden gap of the active layer is smaller than the forbidden gap of the p-type cladding layer and the n-type cladding layer. This energy difference between the forbidden gaps generates energy barriers as the band offset of the valence band and the band offset of the conduction band. These energy barriers, when sufficiently high, can achieve efficient confinement of the injected electrons and holes in the active layer.
With the developments in crystal growth techniques, it has become possible to grow an ultra-thin film having a thickness of several nanometers (nm). Thus, a quantum well (QW) semiconductor laser can be manufactured using an ultra-thin film as an active layer of the laser. In a QW layer of such laser, electrons and holes each have a discrete energy level. As a result, the QW semiconductor laser has advantages such as a decrease in the threshold current density due to an increase in the state densities, emission of laser light having a shorter wavelength, and the like.
One type of QW semiconductor laser is a vertical cavity surface emitting laser (VCSEL). Known in the art are VCSEL devices with GaAs quantum well which emit light in the 850 nanometer range. In such VCSEL, the QW is made from the same material as the substrate, and the various layers, whose thickness is related to wavelength, is able to maintain the minimal mechanical strain without mechanical relaxation. Attempts have been made to use InGaAs or GaAsSb or some combination thereof instead of GaAs in the active layer to provide laser at 1.3 μm [U.S. Pat. No. 6,603,784].
GaInAsN and GaInAsNSb strained QWs have been used for obtaining near-infrared (IR) lasers emitting in wavelengths of 1.3 μm and 1.5 μm, respectively, which are useful for optical fiber communications [Bank et al., IEEE J. Quantum Electron. 43, 773 (2007)].
Also known are laser devices with active regions designed for further extending the wavelengths. These include a bilayer GaInAs/GaAsSb structure and a four-period GaAsN/GaAsSb superlattice for extending the emission wavelength of GaInAs/GaAs based QW lasers, and a Ga(In)AsN/GaAsSb type-II multi-QW W-structure for extending the laser wavelengths toward 1.55 μm [Peter et al., Appl. Phys. Lett. 67, 2639 (1995); and Mawst et al., IEEE J. Sel. Top. Quantum Electron. 14, 979 (2008)].
Additional background art includes U.S. Published Application No. 20100072457.